Gas-Driven Fracturing of Saturated Granular Media
Gas-Driven Fracturing of Saturated Granular Media
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DOI：
10.1103/physrevapplied.8.064029
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发表时间：
2017-12-29
影响因子：
4.6
通讯作者：
Sandnes, Bjornar
中科院分区：
文献类型：
--
作者：
Campbell, James M.;Ozturk, Deren;Sandnes, Bjornar
可变形多孔材料中的多相流在许多地质和岩土工程应用中非常重要；然而，复杂的流动行为使得地下输运过程很难控制-甚至难以表征。在这里，我们研究了限制在Hele-Shaw单元中的湿松散颗粒填料的气体驱动(气动)压裂，并对孔尺度现象和大规模图案的形成进行了深入的分析。这一过程受压力、毛细管力、摩擦力和粘性力之间的复杂相互作用所支配。在低注气速度下，裂缝以粘滑方式生长，并分支形成简单连接的网络。我们观察到一个特征长度尺度的出现--裂缝分支之间的间隔距离--创造了一个明显均匀的空间裂缝密度。我们得出的结论是，明确定义的分离距离是裂缝周围的局部压实锋面使它们保持分离的结果。提出了一个标度参数，它预测裂缝密度是颗粒摩擦、颗粒大小和毛细相互作用的函数。研究了注气速度的影响，发现在临界注气速度以上，系统经历了流态化转变，导致裂缝定向生长，裂缝密度随注气速度的增加而增加。无量纲流态化数F被定义为粘性与摩擦力的比值，我们的实验揭示了F<；1的摩擦状态，其特征是粘滑，与速率无关的增长，并过渡到粘性状态(F>；1)，其特征是多个裂缝分支同时连续增长。
Multiphase flows in deformable porous materials are important in numerous geological and geotechnical applications; however, the complex flow behavior makes subsurface transport processes difficult to control-or even characterize. Here, we study gas-driven (pneumatic) fracturing of a wet unconsolidated granular packing confined in a Hele-Shaw cell, and we present an in-depth analysis of both pore-scale phenomena and large-scale pattern formation. The process is governed by a complex interplay among pressure, capillary, frictional, and viscous forces. At low gas-injection rates, fractures grow in a stick-slip fashion and branch out to form a simply connected network. We observe the emergence of a characteristic length scale-the separation distance between fracture branches-creating an apparent uniform spatial fracture density. We conclude that the well-defined separation distance is the result of local compaction fronts surrounding fractures and keeping them apart. A scaling argument is presented that predicts fracture density as a function of granular friction, grain size, and capillary interactions. We study the influence of the gas-injection rate and find that the system undergoes a fluidization transition above a critical injection rate, resulting in directional growth of the fractures, and a fracture density that increases with an increasing rate. A dimensionless fluidization number F is defined as the ratio of viscous to frictional forces, and our experiments reveal a frictional regime for F < 1 characterized by stick-slip, rate-independent growth, with a transition to a viscous regime (F > 1) characterized by continuous growth in several fracture branches simultaneously.